Paper, for reasons of long familiarity and universal dissemination, is the information storage and transfer medium people are most comfortable and adept with. The most commonly used paper information storage form remains the book and similar objects, and advantages can be obtained by creating the illusion of such objects for those using a computer. In this model, computer objects present their information content as a sequence of regular pages, each potentially displaying a variety of media types.
Browsing information in an electronic medium consists of the repetitive selection and display of portions of that information. These portions of the information are referenced herein as "pages" and the information itself is referenced to as the "document". The "document type" defines how the document is displayed; e.g. an electronic book would have its pages displayed from left to right and a rollerdex from bottom to top.
Reading a document at low speed is easily supported by most hardware architectures. Pages stored in a slow mass storage off-line memory are prefetched into a fast access cache memory before being loaded into the display memory. Prefetching into a cache overcomes delays caused by the slow access time of the off-line mass storage device. As the browsing rate increases, however, the bandwidth of the data paths between the off-line memory, cache and display memories and the memory access times become limiting factors in how quickly new pages of the document can be displayed. Data rates high enough to fulfil the requirement that the user remain unaware of the computer interface can often not be achieved. This leads to uneven and often long delays between the selection of desired material and its display, the delay depending on the page's content. Even when there is a high bandwidth available, complex page structures in the data to be displayed can cause problems for efficient browsing and navigation.
A paper, "An Electronic Book: APTBook", Human-Computer Interaction Conference INTERAC '90, pages 513-519 by Miyazawa et al describes one possible solution to this problem. A technique called "hierarchic compression" is employed for fast browsing. A compressed version of the page content is displayed, showing little detail. As the browsing speed decreases, progressively more details of the page are shown. This method is implemented by storing the book's contents in a fixed hierarchical tree structure. The data is accessed horizontally across the tree, the browsing speed defining the level at which data is accessed. Because the importance of a particular feature of a document is defined by its position in the tree structure, this solution is inflexible and can not easily be adapted to other electronic document structures. This solution also makes assumptions about the logical organization of the electronic document. In some applications the displayed compressed data makes no useful contribution to the display as it is unreadable. In such cases it would be preferable if the compressed data was not displayed at all as it serves only to "clutter" the display. The ability to limit the information displayed is not discussed in the APTBook paper and no mention of how the data structure is related to the memory hierarchy of the browsing system is made.
Other relevant articles include, "Software For Reading Text On Screen", D. J. Pullinger, Human-Computer Interaction, IFIP, 1987 and "Formative Design-Evaluation of SuperBook", D. E. Egan, et al., Bellcore, ACM Transactions on Information Systems, Vol. 7, No. 1. Both articles state the problem well but provide no satisfactory solution to presenting electronically stored data in a book form, which can be as easily and rapidly manipulated by the user as a book.